Electronic Telegram No. 59
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
M.S. 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
CBAT@CFA.HARVARD.EDU (science)
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RADIO TRANSIENT IN SCORPIUS
M. P. Rupen, A. J. Mioduszewski, and V. Dhawan, National Radio Astronomy
Observatory, report strong, on-going radio flares from the microquasar
candidate associated with GSC 7861.1088 = 1RXS J162848.1-415241 (cf. IAUC
7968). Observations with the Very Large Array (VLA) on Jan. 6, 11, and 22
UT gave flux densities at 8.46 GHz of 0.35 +/- 0.08, 13.8 +/- 0.8, and
6.6 +/- 0.4 mJy, respectively. Quasi-simultaneous observations at 4.86 GHz
on the last day gave a flux density of 4.9 +/- 0.3 mJy, implying that the
flux density goes as frequency to the power 0.54 +/- 0.16. Such an inverted
radio spectrum is almost invariably associated with the early, optically
thick stage of a strong radio outburst, and this -- together with the changing
radio flux density -- suggests that the source is currently undergoing a
series of radio flares. The most reliable radio position, based on the
weather and the inverted spectrum (which implies that the source is small,
and presumably close to the binary-star system), is that from the Jan. 22
observations at 4.86 GHz: R.A. = 16h28m47s.283 +/- 0s.003, Decl. =
-41o52'39".05 +/- 0".05 (equinox 2000.0), where the error bars are
statistical, based on a fit using a point source plus a planar background.
This is in excellent agreement with the UCAC2 optical position (Zacharias
et al. 2003, from the Vizier On-line Data Catalog), which gives position
end figures 47s.286, 39".01 (quoted accuracy 0".01). The VLA data show no
obvious signs of extension, with the best limits coming from the Jan. 11
observations, which had a beam size of 3".5 x 0".6, with the long axis
oriented toward p.a. 14 degrees. A Gaussian fit to those data gave a
nominal upper limit to the extent of 0".3 (full-width at half-maximum).
Further VLA observations are planned, and observations at other wavelengths
(particularly radio, x-ray, and optical) are urgently requested.
NOTE: These 'Central Bureau Electronic Telegrams' are intended to
be preliminary announcements of items that later appear in the
formal IAU Circulars. Citations should normally be made to IAUCs
rather than to CBETs.
(C) Copyright 2004 CBAT
2004 January 24 (CBET 59) Daniel W. E. Green